Synthesis of chiral α-diarylacetic esters by stereospecific 1,2-aryl migration promoted by in situ generated acetals from benzoins

Article abstract of DOI:10.1021/ol500292c

A simple protocol for the synthesis of α-diarylacetic esters from benzoins is described. In situ generated acetal assists rapid 1,2-aryl migration in a stereospecific manner, paving the way to make enantioenriched α-diarylacetic esters from easily accessi

Full text of DOI:10.1021/ol500292c

Letter
pubs.acs.org/OrgLett
Synthesis of Chiral α‑Diarylacetic Esters by Stereospecific 1,2-Aryl
Migration Promoted by in Situ Generated Acetals from Benzoins
Raveendra Babu Kothapalli, Ramana Niddana, and Rengarajan Balamurugan*
School of Chemistry, University of Hyderabad, Dr. C. R. Rao Road, Gachibowli, Hyderabad 500046, India
S
* Supporting Information
ABSTRACT: A simple protocol for the synthesis of α-diarylacetic
esters from benzoins is described. In situ generated acetal assists rapid
1,2-aryl migration in a stereospecific manner, paving the way to make
enantioenriched α-diarylacetic esters from easily accessible enantio-
pure benzoins.
he chiral gem-diarylalkyl moiety is present in many
Our strategy essentially involves in situ generation of acetals
1
Tpharmaceutically and biologically important compounds. from benzoins using triethyl orthoformate in the presence of
TfOH and their rapid conversion into α-diarylacetates (Scheme
For example, this moiety is the key structural feature of
muscarinic antagonists like fesoterodine1b and tolterodine
(Detrol),1c,d antidepressants such as sertraline (Zoloft)1e and
nomifensine,1f and phosphodiesterase type 4 inhibitor CDP-
840.1g Among the chiral gem-diarylalkyl compounds, making
enantiomerically pure α-diaryl carbonyl compounds is a really
challenging task due to the associated epimerization issues.
Straightforward enantioselective protonation of enolates, which
is used to introduce chirality at the α-position of a carbonyl
function, cannot be applied to racemic α-diaryl carbonyl
compounds as it is difficult to differentiate the faces of their
enolates. There are quite a few methods available for the
synthesis of racemic unsymmetrical α-diaryl carbonyl com-
pounds.2 Several of them are based on the transition-metal-
catalyzed α-arylation of α-aryl carbonyl compounds. Although
the asymmetric version of transition-metal-catalyzed α-
arylation3 has seen remarkable development in recent years, it
is limited to the synthesis of enantiopure α-monoaryl carbonyl
compounds only.
Very few reports are available for the synthesis of
enantioenriched α-diaryl carbonyl compounds.4−6 Among
them, a majorly explored strategy is the 1,2-addition of aryl
boronic acids to α-ketoesters or α-diketones using chiral
rhodium or ruthenium catalysts.4 This reaction results in
enantioenriched α-hydroxydiaryl carbonyl compounds. Mar-
uoka and co-workers have developed a strategy for generating a
chiral all-carbon quaternary center α to carbonyl by an
interesting Brønsted acid-catalyzed insertion of aryldiazoacetate
into the sp2-carbon−CHO bond.5 In this reaction, the chirality
is transferred through (−)-phenylmenthyl auxiliary attached to
the aryldiazo substrate. Davies and co-workers have reported a
rhodium-catalyzed asymmetric insertion of aryl diazoacetate
into the C−H bond of 1,4-cyclohexadiene and subsequent
oxidation of the product using DDQ to gem-diarylacetates.6
This method, however, is applicable to make enantioenriched
α-diarylacetates in which one of the aryl groups is compulsorily
phenyl. Herein, we report a facile strategy for obtaining racemic
as well as both isomers of α-diarylacetates.
1, eq 1). Olah and co-workers have reported benzannulation of
Scheme 1
benzoin under the same conditions in the absence of triethyl
orthoformate (Scheme 1, eq 2).7 Thus, the 1,2-aryl migration is
facilitated by the in situ formed acetal in the reactions presented
in this paper. The reason is, upon 1,2-migration of aryl group,
the generated carbocation is stabilized effectively by the acetal
oxygen atoms. 1,2-Aryl migration has been known to provide
opportunities to synthesize compounds which are difficult to
make.8 Many Lewis acids such as salts of Zn, Sn, Co, Hg, Pd,
Sb, Bi, and Fe (MXn) can promote the 1,2-aryl shift in α-
haloalkylaryl acetals to α-arylalkanoic acids at reflux temper-
atures.9 In these reactions, preformed acetal was employed and
the affinity of metal toward halide promoted the 1,2-aryl
migration. Under strong Brønsted/Lewis acidic conditions, it is
known that aldehydes and ketones are converted in to
corresponding acetals in the presence of trialkyl orthofor-
mates.10 Miroslav and co-workers have used in situ formed
acetal for the preparation of methyl 2-(4-ethylphenyl)-2-
methylpropionate from 1-(4-ethylphenyl)-2-hydroxy-2-methyl-
propan-1-one.9a However, synthesis of enantioenriched gem-
Received: October 16, 2013
Published: February 21, 2014
© 2014 American Chemical Society
1278
dx.doi.org/10.1021/ol500292c | Org. Lett. 2014, 16, 1278−1281

Organic Letters
Letter
diarylacetates have not been explored in the above-mentioned
reactions involving 1,2-aryl migration.
Table 2. Scope of in Situ Formed Acetal-Assisted 1,2-Aryl
Migration
Optimization of reaction conditions was carried out using
different Brønsted acids taking benzoin as the starting material.
Table 1 summarizes the results of these reactions. Among the
Table 1. Screening of Different Brønsted Acids in the
Conversion of Benzoin into Diphenylacetate Derivatives
a
entry
1
Ar1
Ar2
2
yield (%)
1
1a
1b
1c
1d
1e
1f
Ph
Ph
2a
2b
2c
2d
2e
2f
97
90
88
78
98
97
80
99
95
97
87
92
90
83
95
98
88
91
86
90
76
85
2
4-FC6H4
4-FC6H4
4-ClC6H4
4-BrC6H4
2-ClC6H4
4-ClC6H4
2,4-Cl2C6H3
Ph
3
4-ClC6H4
4
4-BrC6H4
5
Ph
a
entry
Brønsted acid
R
time
1 h
yield (%)
6
Ph
1
CH3SO3H
TFA
Et
89
7
1g
1h
1i
Ph
2g
2h
2i
2
Et
24 h
79
8
4-MeC6H4
3
PTSA
Et
24 h
24 h
24 h
24 h
24 h
24 h
15 min
15 min
4 h
70
9
4-MeC6H4
4-ClC6H4
3-FC6H4
4-ClC6H4
4-FC6H4
4-FC6H4
Ph
4
HClO4
Et
11
10
11
12
13
14
15
16
17
18
19
20
21
22
1j
4-MeC6H4
2j
5
(+)-CSA
HCl
Et
49
1k
1l
4-(OMe)C6H4
4-(OMe)C6H4
4-ClC6H4
2k
2l
6
Et
trace
NR
NR
97
95
7
Amberlyst 15
Amberlite IR 120
TfOH
Et
1m
1n
1o
1p
1q
1r
1s
1t
2m
2n
2o
2p
2q
2r
2s
2t
8
Et
3,4-(OMe)2C6H3
3,4-(OMe)2C6H3
3,4-(OCH2O)C6H3
3,4-(OCH2O)C6H3
3,4-(OCH2O)C6H3
4-(TBDPSO)C6H4
4-(TBDMSO)C6H4
4-(OH)C6H4
4-(OAc)C6H4
9
Et
Me
Me
Me
4-ClC6H4
Ph
10
11
12
TfOH
b
TfOH
6
4-FC6H4
4-ClC6H4
C6H5
c
TfOH
27 h
61
a
b
c
Isolated yield. 10 mol %. 50 mol %. NR: no reaction.
C6H5
1u
1v
C6H5
2u
2v
Brønsted acids studied, triflic acid was found to be the best in
the presence of 3 equiv of triethyl orthoformate, and the
reaction was complete in 15 min at room temperature (Table 1,
entry 9). With substoichiometric amount of triflic acid the
reaction resulted in poor yields of the product (Table 1, entries
11 and 12).
The optimized reaction conditions (Table 1, entry 9) were
employed in the 1,2-aryl migration reactions of different
benzoins. The results are presented in Table 2. Benzoins
employed in this study were prepared either by trivial benzoin
condensation or a two-step process involving (2-aryl-1,3-
dithan-2-yl)lithium addition to arylaldehydes followed by
deprotection of dithiane moiety.11 Benzoins having different
substituents like F, Cl, Br, Me, OMe, OCH2O, OTBDPS,
OTBDMS, OH, and OAc on aryl rings were employed. The
reactions were fast and completed in less than 20 min. The
reactions were clean and the yields were generally excellent. To
extend this methodology, 1,2-alkyl migration was probed using
the substrates 1-hydroxy-1-phenylpentan-2-one (1w)11 and 1-
hydroxy-3-methyl-1-phenylbutan-2-one (1x).11 Unfortunately,
both reactions resulted in complex reaction mixtures.
We then focused our attention on the synthesis of
enantiomerically pure benzoins to check whether the 1,2-aryl
migration in the present reaction occurs at the same time as the
OH leaves in the form of water. For the synthesis of optically
pure benzoins a new protocol was designed. The benzylic
alcohol derivatives obtained in the reaction of (2-aryl-1,3-
dithan-2-yl)lithium with arylaldehydes were oxidized to their
corresponding ketones. CBS reduction was carried out on the
ketone function and deprotection of dithiane in the resulting
alcohols generated optically active benzoins with high
enantiomeric purities.11 This way, both isomers of benzoins
could be obtained using (S)-DPP and (R)-DPP in high ee.12
This new protocol for the synthesis of enantiopure benzoins is
very straightforward. Enantioselective CBS reduction of 2-acyl-
C6H5
a
Isolated yield.
1,3-dithianes using B-Ph oxazaborolidine derivative as the
catalyst has been reported to show good enantioselectivities.13
In these substrates, the bulkiness is imparted by the dithiane
moiety and good enantioselectivities were obtained only when
the acyl group is kept less bulky like acetyl. When the acyl
group is propanoyl the enantioselectivity reduced considerably.
However, using classical B-Me oxazaborolidine as catalyst, we
were delighted to observe very good enantioselectivities with
our substrates having bulky aryloyl as the acyl component.
Benzoins (R)-1f, (R)-1l, and (S)-1l have already been reported.
By analogy, the absolute configurations of other benzoins were
assigned.
Both (R)- and (S)-benzoins were subjected to in situ formed
acetal-assisted 1,2-aryl migration separately. Interestingly, the
aryl migration took place in a stereospecific way to result in
enantioenriched ethyl α-diarylacetates in excellent yields
(Tables 3 and 4). That is, the aryl migration took place from
back side of the leaving OH group in a concerted process.14
The anchimeric assistance of aryl group and stabilization of
carbocationic center by acetal function favor this stereospecific
1,2-aryl migration. Except in the reaction of (S)-1m, the ee of
the products are the same or slightly less than that of the
starting benzoins. Whenever there was an electron-withdrawing
fluoro substituent present in the aryl ring, the ee was slightly
reduced. This effect was greater with substrate 1m, which
contains p-F-phenyl and p-Cl-phenyl groups.
In summary, a simple protocol involving in situ generated
acetal-assisted 1,2-aryl migration for the synthesis of gem-
diarylacetates has been developed. Using this method,
enantioenriched α-diarylacetates which are otherwise difficult
to make could be prepared from easily accessible enantiomeri-
1279
dx.doi.org/10.1021/ol500292c | Org. Lett. 2014, 16, 1278−1281

Organic Letters
Letter
R. P.; Warrellow, G. J.; Eaton, M. A. W.; Boyd, E. C.; Head, J. C.;
Porter, J. R.; Brown, J. A.; Reuberson, J. T.; Hutchinson, B.; Turner, P.;
Boyce, B.; Barnes, D.; Mason, B.; Cannell, A.; Taylor, R. J.; Zomaya,
A.; Millican, A.; Leonard, J.; Morphy, R.; Wales, M.; Perry, M.; Allen,
R. A.; Gozzard, N.; Hughes, B.; Higgs, G. Bioorg. Med. Chem. Lett.
2002, 12, 1451. For recent methods for the synthesis of
enantioenriched gem-diaryl compounds, see: (h) Foschi, F.;
Tagliabue, A.; Mihali, V.; Pilati, T.; Pecnikaj, I.; Penso, M. Org. Lett.
2013, 15, 3686. (i) Spahn, E.; Albright, A.; Shevlin, M.; Pauli, L.;
Pfaltz, A.; Gawley, R. E. J. Org. Chem. 2013, 78, 2731. (j) Ohmiya, H.;
Makida, Y.; Li, D.; Tanabe, M.; Sawamura, M. J. Am. Chem. Soc. 2010,
132, 879. (k) Gustafson, J. L.; Sigman, M. S.; Miller, S. J. Org. Lett.
Table 3. Preparation of Enantiopure α-Diarylacetic Esters
from (R)-Benzoins
a
entry
1
ee (%)
2
yield (%)
ee (%)
1
2
3
4
5
(R)-1f
(R)-1k
(R)-1l
(R)-1q
(R)-1r
91
87
92
98
95
(S)-2f
(S)-2k
(S)-2l
(R)-2q
(R)-2r
87
90
88
87
88
88
87
90
93
93
2010, 12, 2794. (l) Mazuela, J.; Verendel, J. J.; Coll, M.; Schaffner, B.;
̈
́ ́
Borner, A.; Andersson, P. G.; Pamies, O.; Dieguez, M. J. Am. Chem.
̈
a
Soc. 2009, 131, 12344. (m) Luan, X.; Mariz, R.; Robert, C.; Gatti, M.;
Blumentritt, S.; Linden, A.; Dorta, R. Org. Lett. 2008, 10, 5569.
(2) For selected reviews, see: (a) Baudoin, O. Chem. Soc. Rev. 2011,
40, 4902. (b) Johansson, C.; Colacot, T. Angew. Chem., Int. Ed. 2010,
49, 676. (c) Bellina, F.; Rossi, R. Chem. Rev. 2010, 110, 1082.
(d) Culkin, D.; Hartwig, J. F. Acc. Chem. Res. 2003, 36, 234. For
syntheses of gem-diaryl compounds, see: (e) Auvil, T. J.; So, S. S.;
Mattson, A. E. Angew. Chem., Int. Ed. 2013, 52, 11317. (f) Lessi, M.;
Masini, T.; Nucara, L.; Bellina, F.; Rossi, R. Adv. Synth. Catal. 2011,
353, 501. (g) Kitamura, M.; Miyagawa, S.; Okauchi, T. Tetrahedron
Lett. 2011, 52, 3158. (h) Peng, C.; Zhang, W.; Yan, G.; Wang, J. Org.
Lett. 2009, 11, 1667. (i) Martin, R.; Buchwald, S. Angew. Chem., Int. Ed.
2007, 46, 7236. (j) Beare, N.; Hartwig, J. F. J. Org. Chem. 2002, 67,
541. (k) Gaertzen, O.; Buchwald, S. J. Org. Chem. 2002, 67, 465.
(l) Churruca, F.; SanMartin, R.; Tellitu, I.; Dominguez, E. Org. Lett.
2002, 4, 1591. (m) Moradi, W.; Buchwald, S. J. Am. Chem. Soc. 2001,
123, 7996. (n) Stauffer, S.; Beare, N.; Stambuli, J.; Hartwig, J. F. J. Am.
Chem. Soc. 2001, 123, 4641. (o) Old, D.; Wolfe, J.; Buchwald, S. J. Am.
Chem. Soc. 1998, 120, 9722.
Isolated yield.
Table 4. Preparation of Enantiopure α-Diarylacetic Esters
from (S)-Benzoins
a
entry
1
ee (%)
2
yield (%)
ee (%)
1
2
3
4
5
(S)-1f
(S)-1l
(S)-1m
(S)-1q
(S)-1r
99
96
99
98
97
(R)-2f
(R)-2l
(S)-2m
(S)-2q
(S)-2r
96
93
93
96
85
93
91
85
92
96
a
Isolated yield.
(3) For reviews on asymmetric α-arylation, see: (a) Mazet, C. Synlett
2012, 1999. (b) Burtoloso, A. C. B. Synlett 2009, 320. For asymmetric
α-arylation, see: (c) Tomohara, K.; Yoshimura, T.; Hyakutake, R.;
Yang, P.; Kawabata, T. J. Am. Chem. Soc. 2013, 135, 13294. (d) Choi,
J.; Fu, G. C. J. Am. Chem. Soc. 2012, 134, 9102. (e) Nareddy, P.;
cally pure benzoins. Optically active α-diarylacetates could serve
as valuable building blocks for the synthesis of interesting
molecular skeletons.
ASSOCIATED CONTENT
* Supporting Information
Mantilli, L.; Guen
3826. (f) Lee, J.-W.; List, B. J. Am. Chem. Soc. 2012, 134, 18245.
́ ́
ee, L.; Mazet, C. Angew. Chem., Int. Ed. 2012, 51,
■
S
(g) Carroll, M. P.; Muller-Bunz, H.; Guiry, P. J. Chem. Commun. 2012,
̈
Complete experimental details, HPLC traces of enantiopure
compounds, and spectra. This material is available free of
charge via the Internet at http://pubs.acs.org.
48, 11142. (h) Ge, S.; Hartwig, J. F. J. Am. Chem. Soc. 2011, 133,
16330. (i) Huang, Z.; Liu, Z.; Zhou, J. J. Am. Chem. Soc. 2011, 133,
15882. (j) Liao, X.; Stanley, L. M.; Hartwig, J. F. J. Am. Chem. Soc.
2011, 133, 2088. (k) Rendina, V. L.; Moebius, D. C.; Kingsbury, J. S.
Org. Lett. 2011, 13, 2004. (l) Luan, X.; Wu, L.; Drinkel, E.; Mariz, R.;
Gatti, M.; Dorta, R. Org. Lett. 2010, 12, 1912. (m) Liao, X.; Weng, Z.;
Hartwig, J. F. J. Am. Chem. Soc. 2008, 130, 195. (n) Luan, X.; Mariz,
R.; Robert, C.; Gatti, M.; Blumentritt, S.; Linden, A.; Dorta, R. Org.
Lett. 2008, 10, 5569. (o) Chen, G.; Kwong, F. Y.; Chan, H. O.; Yu, W.-
Y.; Chan, A. S. C. Chem. Commun. 2006, 1413. (p) Hamada, T.;
Chieffi, A.; Åhman, J.; Buchwald, S. L. J. Am. Chem. Soc. 2002, 124,
1261. (q) Spielvogel, D. J.; Buchwald, S. L. J. Am. Chem. Soc. 2002,
124, 3500.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: rbsc@uohyd.ernet.in.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Department of Science and Technology (DST),
India, and UGC UPE-II for financial support. R.B.K. and R.N.
thank the Council of Scientific and Industrial Research (CSIR),
India, for senior research fellowships.
(4) (a) Zhu, T.-S.; Jin, S.-S.; Xu, M.-H. Angew. Chem., Int. Ed. 2012,
51, 780. (b) Cai, F.; Pu, X.; Qi, X.; Lynch, V.; Radha, A.; Ready, J. M. J.
Am. Chem. Soc. 2011, 133, 18066. (c) Yamamoto, Y.; Shirai, T.;
Watanabe, M.; Kurihara, K.; Miyaura, N. Molecules 2011, 16, 5020.
(d) Duan, H.-F.; Xie, J.-H.; Qiao, X.-C.; Wang, L.-X.; Zhou, Q.-L.
Angew. Chem., Int. Ed. 2008, 47, 4351.
(5) Hashimoto, T.; Naganawa, Y.; Maruoka, K. J. Am. Chem. Soc.
2008, 130, 2434.
(6) Davies, H. M. L.; Stafford, D. G.; Hansen, T. Org. Lett. 1999, 1,
233.
(7) Olah, G. A.; Wu, A.-h. J. Org. Chem. 1991, 56, 2531.
REFERENCES
■
(1) For a recent review on chiral 1,1-diaryl compounds as
pharmacophores, see: (a) Ameen, D.; Snape, T. J. Med. Chem.
Commun. 2013, 4, 893. (b) McKeage, K.; Keating, G. M. Drugs 2009,
69 (6), 731. (c) Malhotra, B.; Gandelman, K.; Sachse, R.; Wood, N.;
Michel, M. C. Curr. Med. Chem. 2009, 16, 4481. (d) Nilvebrant, L.;
Andersson, K.-E.; Gilberg, P.-G.; Stahl, M.; Sparf, B. Eur. J. Pharmacol.
1997, 327, 195. (e) Welch, W. M.; Kraska, A. R.; Sarges, R.; Koe, B. K.
́
(8) For very recent literature on 1,2-aryl shift, see: (a) Gutierrez-
́
Bonet, A.; Flores-Gasper, A.; Martin, R. J. Am. Chem. Soc. 2013, 135,
12576. (b) Liu, L.; Lu, H.; Wang, H.; Yang, C.; Zhang, X.; Zhang-
Negrerie, D.; Du, Y.; Zhao, K. Org. Lett. 2013, 15, 2906. (c) Lie, X.;
J. Med. Chem. 1984, 27, 1508. (f) Zar
́
a-Kaczian, E.; Gyorgy, L.; Deak,
́
́
̈
G.; Seregi, A.; Doda, M. J. Med. Chem. 1986, 29, 1189. (g) Alexander,
́
1280
dx.doi.org/10.1021/ol500292c | Org. Lett. 2014, 16, 1278−1281

Organic Letters
Letter
Xiong, F.; Huang, X.; Xu, L.; Li, P.; Wu, X. Angew. Chem., Int. Ed.
2013, 52, 6962. (d) Farid, U.; Malmedy, F.; Claveau, R.; Albers, L.;
Wirth, T. Angew. Chem., Int. Ed. Engl. 2013, 52, 7018. (e) Guer
́
ard, K.
C.; Guerinot, A.; Bouchard-Aubin, C.; Menard, M.-A.; Lepage, M.;
́
́
Beaulieu, A.; Canesi, S. J. Org. Chem. 2012, 77, 2121. (f) Singh, F. V.;
Rehbein, J.; Wirth, T. ChemistryOpen 2012, 1, 245.
(9) (a) Miroslav, V.; Andrej, B.; Ambroz, A.; Gabriela, A.; Ivan, V.
PCT/SK2003/000022; WO 2004/046080. (b) Pol, A. V.; Sudalai, A.;
Sonawane, H. R. Indian J. Chem. 1998, 37B, 239. (c) Kumar, A.; Rane,
R. A.; Ravindran, V. K.; Dike, S. Y. Synth. Commun. 1997, 27, 1133.
(d) Sonawane, H. R.; Nanjundiah, B. S.; Kulkarni, D. G.; Ahuja, J. R.
Tetrahedron 1988, 44, 7319. (e) Yamauchi, T.; Hattori, K.; Nakao, K.;
Tamaki, K. Bull. Chem. Soc. Jpn. 1987, 60, 4015. (f) Honda, Y.; Ori, A.;
Tsuchihashi, G.-i. Bull. Chem. Soc. Jpn. 1987, 60, 1027. (g) Yamauchi,
T.; Hattori, K.; Nakao, K.; Tamaki, K. Bull. Chem. Soc. Jpn. 1987, 60,
4015. (h) Castaldi, G.; Belli, A.; Uggeri, F.; Giordano, C. J. Org. Chem.
1983, 48, 4658. (i) Giordano, C.; Castaldi, G.; Casagrande, F.; Abis, L.
Tetrahedron Lett. 1982, 23, 1385. (j) Giordano, C.; Castaldi, G.;
Casagrande, F.; Belli, A. J. Chem. Soc., Perkin Trans. 1 1982, 2575.
(10) (a) LaMattina, J. L.; Muse, D. E. J. Org. Chem. 1987, 52, 3479.
(b) Williams, D. B. G.; Lawton, M. C. Green Chem. 2008, 10, 914.
(c) Hamada, N.; Kazahaya, K.; Shimizu, H.; Sato, T. Synlett 2004,
1074. (d) Kumar, R.; Chakraborti, A. K. Tetrahedron Lett. 2005, 46,
8319.
(11) See the Supporting Information for details.
(12) (a) Corey, E. J.; Bakshi, R. K.; Shibata, S. J. Am. Chem. Soc.
1987, 109, 5551. (b) Corey, E. J.; Bakshi, R. K.; Shibata, S.; Chen, C.
P.; Singh, V. K. J. Am. Chem. Soc. 1987, 109, 7925.
(13) DeNinno, M. P.; Perner, R. J.; Lijewski, L. Tetrahedron Lett.
1990, 31, 7415.
(14) For stereospecific aryl migrations, see: (a) Tomohara, K.;
Yoshimura, T.; Hyakutake, R.; Yang, P.; Kawabata, T. J. Am. Chem. Soc.
2013, 135, 13294. (b) Fournier, A. M.; Brown, R. A.; Farnaby, W.;
Ondozabal, H. M.; Clayden, J. Org. Lett. 2010, 12, 2222. (c) Clayden,
J.; Farnaby, W.; Grainger, D. M.; Hennecke, U.; Mancinelli, M.;
Tetlow, D. J.; Hillier, I. H.; Vincent, M. A. J. Am. Chem. Soc. 2009, 131,
3410. (d) Clayden, J.; Dufour, J.; Grainger, D. M.; Helliwell, M. J. Am.
Chem. Soc. 2007, 129, 7488. (e) Amrein, S.; Bossart, M.; Vasella, T.;
Studer, A. J. Org. Chem. 2000, 65, 4281. (f) Sonawane, H. R.;
Nanjundiah, B. S.; Kulkarni, D. G.; Abuja, J. R. Tetrahedron: Asymmetry
1991, 2, 251.
1281
dx.doi.org/10.1021/ol500292c | Org. Lett. 2014, 16, 1278−1281